The present invention relates to the selective internal platinization for porous aluminosilicates.
Natural and synthetic aluminosilicates are of practical importance as molecular sieves and catalysts. Synthetic zeolites modified with nonframework metals are among those materials that have been intensively studied with regard to cation-exchange properties, metal ion localization, and catalysis..sup.[1-3] Transition metal ion exchanged zeolites have been found to be particularly efficient for various catalytic reactions. For example, oxidation of propylene by cupric ion exchanged zeolite Y,.sup.[4] cyclodimerization of butadiene,.sup.[5] and the catalytic oxidation of ethylene by Pd.sup.2+ /Cu.sup.2+ metal ions of zeolite Y.sup.[6] have been described. Platinized zeolite L has been used in nonacidic catalytic re-forming and dehydro-cyclization reactions..sup.[7] Much of this chemistry has focused on creating encapsulated transition-metal catalysts. The principal role of the zeolite framework is to provide a compartmentalized environment, the metal being immobilized within the microporous structure. Molecules that poison the catalyst, if they are sufficiently large, may be size-excluded from the zeolite pore structure..sup.[8] With smaller pore zeolites such as ZSM-5 andZSM-11, reactant or product selectivity in a metal-catalyzed reaction is achieved by forcing the reactant molecule to pass through the intracrystalline volume in order to contact the metal. Good selectivity is usually obtained by poisoning the metal sites on the external surface with bulky ligands that are too large to access the internal metal sites..sup.[9-11] Intrazeolite reactions are of course highly desirable in shape-selective catalysis since such selectivity may exceed that possible with liquid-phase catalysts..sup.[12]
Some of the charge-balancing counterions of the anionic zeolite framework are readily exchangeable, and the usual route is to introduce metals by equilibration of cationic metal complexes or metal ions with a suspension of the zeolite. Thus Pt and Pd are easily loaded into the large-pore zeolites Y and L by exchanging Na.sup.+ or NH.sub.4.sup.+ ions with Pt(NH.sub.3).sub.4.sup.2+ or Pd(NH.sub.3).sub.4.sup.2+..sup.[8,13] Different loadings of noble metal are readily obtained by this ion-exchange method. Transmission electron microscopy and gas adsorption measurements have shown .sup.[8,13] that the bulk of the metal deposited is highly dispersed and lies within the zeolite pore structure. In one embodiment, the present invention involves preparation, using a neutral platinum complex, of zeolite L powders that contain platinum metal exclusively on their internal or inner surfaces. Two sensitive chemical tests are described herein that can differentiate between zeolites platinized both inside and outside and those platinized only on the inside. This is the first time that chemical tests have established the absence of metal clusters or particles on the outer surfaces of a metalated zeolite. These platinized zeolite L powders are part of an integrated system for light-driven vectorial electron transport and hydrogen evolution..sup.[14]